A process for producing an aromatic polyimide separating membrane is disclosed, for example, in Japanese Patent Application Laid-Open (Kokai) No. 49-45152. In this process, a homogeneous solution containing an aromatic polyamic acid dissolved in a solvent is used as a dope solution for forming a membrane. A liquid mixture containing an imide-cyclization composition prepared from a lower aliphatic carboxylic acid or its anhydride and a tertiary amine is used as a coagulating liquid for the dope solution. A thin layer of the dope solution is formed on a smooth surface of a membrane-forming substrate by means of a wet membrane-forming method and, then, is coagulated by the coagulating liquid while the polyamic acid is converted to the corresponding polyimide, so as to provide a polyimide asymmetric membrane.
In the above-mentioned membrane-forming method, however, since the coagulation of the thin layer of the dope solution and the imide-cyclization of the polyamic acid in the dope solution are simultaneously carried out, it is difficult to obtain an aromatic polyimide membrane having a satisfactory, uniform gas-separating property with satisfactory reproductivity. That is, the products of the above-mentioned conventional membrane-forming method sometimes exhibit an uneven gas-separating property and/or an unsatisfactory gas-permeating rate.
Accordingly, there has been a great demand for a process of producing an aromatic polyimide membrane having excellent gas-separating properties with excellent reproductivity.